Hammering sheet metal into shape

The tool at the pressing plant resonantly pounds the sheet metal, ejecting the newly formed vehicle hood moments later. Although this operation runs like clockwork on the production line, it caused the developers of the metal-forming equipment many a headache, since sheet metal springs back (unbends elastically) when the press is opened.

The shaped part for a fender, for example, can differ from the forming tool by up to several centimeters. 'To a large extent, the experts understand the mechanical behavior of sheet metal and modify the geometry of the tool accordingly,' says Dr. Winfried Schmitt of the Fraunhofer Institute for Mechanics of Materials IWM. 'Yet even they need several costly and time-consuming redesign cycles to achieve the required dimensional precision.' The process becomes significantly more complex when forming thin sheets made of higher strength steel or light metals such as aluminum or magnesium alloys, since there is little experience of working with these modern materials. To shorten the route to the perfect shape, companies such as ThyssenKrupp, Müller-Weingarten and Karmann call on the knowledge, measurement techniques and calculations of experts at the Fraunhofer Competence Center for Component Simulation.

In the processing of sheet metal, especially in the case of complex shapes, there are many different forces at work: An area of the sheet that has been stretched may need to be compressed shortly afterwards. Other areas may be subject to different types of stress at slightly different times. Thin sheet metal, which is increasingly used in lightweight construction, is especially critical, since it is more prone to fracturing than conventional sheet metal. 'It is precisely this varying temporal and spatial interplay of forces that was scarcely considered in previous simulation techniques,' emphasizes Schmitt. 'We therefore investigate the behavior of the sheet metal in combined tensile and compression tests. Once we have this reference data and the geometry of the pressing tool, the behavior of the entire component can then be reliably predicted by computer.'

Similar problems occur elsewhere, in a completely different industry. The service life of electrical and electronic devices is often limited by the quality of the miniature plug-in connections. In this instance, the question is: What is the best way to form a copper connector, to allow for maximum elasticity and ensure reliable electrical contact? The computer model even allows designers to predict the gradual wear on metal surfaces through repeated connection cycles.